1. Field of the Invention
The present invention relates to a Local Area Network (LAN), and more particularly, to a wireless Local Area Network (LAN) system and method based on the IEEE802.11 (Ethernet) standard.
2. Description of the Related Art
Generally, a wireless Local Area Network (LAN) system connects personal or public LANs wirelessly to each other so that users can exchange data using mobile terminals, hand phones, PDAs, laptop computers, etc. Particularly, the IEEE 802.11.g standard proposes an integrated processing system that integrates a Direct Sequence Spread Spectrum (DSSS) signal, a Complementary Code Keying (CCK) signal, and the Orthogonal Frequency Division Multiplexing (OFDM) signal, thereby allowing these signals to be processed in one system. The 802.11b standard uses DSSS to disperse the data frame signal over a relatively wide (approximately 30 MHz) portion of the 2.4 GHz frequency band. In order to actually spread the signal, an 802.11 transmitter combines the PPDU with a spreading sequence through the use of a binary adder. The spreading sequence is a binary code. For 1 Mbps and 2 Mbps operation, the spreading code is the 11-chip Barker sequence, which is 10110111000. 5.5 Mbps and 11 Mbps operation of 802.11b doesn't use the Barker sequence, and instead uses complementary code keying (CCK) to provide the spreading sequences at these higher data rates. Complementary Code Keying, uses a set of 64 eight-bit code words to encode data for 5.5 and 11 Mbps data rates in the 2.4 GHz band of 802.11b wireless networking. The code words have unique mathematical properties that allow them to be correctly distinguished from one another by a receiver even in the presence of substantial noise and multipath interference. CCK works in conjunction with the DSSS technology that is specified in the original 802.11 standard. CCK derives a different spreading code based on fairly complex functions depending on the pattern of bits being sent. The modulator simply refers to a table for the spreading sequence that corresponds to the pattern of data bits being sent. This is necessary to obtain the most efficient processing of the data in order to achieve the higher data rates. CCK applies sophisticated mathematical formulas to the DSSS codes, permitting the codes to represent a greater volume of information per clock cycle. The transmitter can then send multiple bits of information with each DSSS code, enough to make possible the 11 Mbps transmission rate. Generally, the DSSS signal is transmitted or received at a transmission rate of approximately 1 Mbps on a 2.4 GHz band, and the CCK signal is transmitted or received at a transmission rate of maximally 11 Mbps on the 2.4 GHz band.
The OFDM signal is generally transmitted or received at a transmission rate of maximally 54 Mbps on a high frequency band of 5.4 GHz as defined in the IEEE 802.11.a standard. However, since the integrated processing system proposed in the IEEE 802.11.g standard should use an RF module, a frequency band of approximately 20 MHz among the 2.4 GHz band (2.4 GHz-2.4835 GHz) was proposed for this system.
A general technique and system for transmitting the DSSS signal, the CCK signal, or the OFDM signal are disclosed in detail in U.S. Pat. No. 6,256,508 and U.S. Pat. Publication No. 2002/0159422.
FIG. 1 is a timing diagram showing general structures of a DSSS signal and an OFDM signal.
To determine whether an input signal is a DSSS signal or an OFDM signal, a preamble processing step is performed. Referring to FIG. 1, the DSSS signal and CCK signal are subjected to a preamble processing step (“Signal Detect”) for 56 μs The OFDM signal is subjected to a preamble processing step for 16 μs.
However, while the integrated processing system according to the IEEE 802.11.g standard is capable of transmitting a packet signal (for example, as a DSSS/CCK signal or as a OFDM signal), it may have difficulty distinguishing whether a received signal is a DSSS/CCK signal or an OFDM signal.
The conventional process for receiving DSSS/CCK and OFDM signals proceeds as follows. If a signal existing on an allocated channel in the 2.4 GHz-2.4835 GHz frequency band is extracted using an RF module, the extracted signal is amplified to have a valid amplitude required for signal determination, and is then subjected to a preamble processing step for determining whether the signal is a DSSS/CCK signal or an OFDM signal. The received signal, whose standard is being determined, is demodulated in a next unit and is transferred to a user via a Media Access Control (MAC) layer.
For receiving various signals in an integrated DSSS/CCK and OFDM system, when an AGC-processed analog signal is sampled and converted into a digital signal, a DSSS/CCK signal with a rate of 1 MHz should be sampled in synchronization to a clock signal of 11 MHz/22 MHz/44 MHz, and the OFDM signal with a rate of 20 MHz should be sampled in synchronization to a clock signal of 20 MHz/40 MHz/80 MHz. When determining the standard of a received signal sampled using different sampling clocks, the preamble processing of the received signal has to be completed in a defined preamble period of time, for example, in a preamble time period of 16 μs in a case of an OFDM signal.